1. Field of the Invention
The present invention relates to an electron gun for a color cathode ray tube (CRT), and, more particularly, to an in-line electron gun for a color CRT having improved electrodes that form at least one quadrupole lens.
2. Description of the Related Art
A typical electron gun for a color CRT is installed in a neck portion of the CRT and emits thermal electrons. The performance of the color CRT depends on the state of electron beams emitted from the electron gun and landing on a fluorescent film. Thus, numerous electron guns have been developed to improve focus properties and reduce aberration of an electron lens so that the electron beams emitted from the electron gun accurately land on phosphor dots of the fluorescent film. In particular, to reduce the total length of a CRT, the deflection angle of the electron beams is increased and the length of the electron gun is reduced. Since the focusing distance of an electron beam landing on a peripheral portion of a fluorescent film is larger than that of an electron beam landing on a central portion of the fluorescent film, the focus of the electron beams at the peripheral portion of a screen is inferior to the focus at the central portion of the fluorescent film.
As deflection angle increases, an incident angle of an electron beam with respect to the fluorescent film decreases. Accordingly, the distortion of the electron beam increases exponentially with deflection angle so that the diameter of a spot produced by an electron beam landing on the fluorescent film increases. The electron beam emitted from the electron gun is converged throughout the entire surface of a screen by a non-uniform electric field including a pincushion horizontal deflection electric field and a barrel vertical deflection electric field generated by a deflection electric field of the electron gun. This non-uniform electric field diverges the beam horizontally and focuses the beam vertically, forming a horizontally elongated beam at the periphery of a screen, lowering resolution. Of three in-line electron beams lying in a horizontal plane and produced by an electron gun for a color CRT, the two outside electron beams are more affected by astigmatism than is the central electron beam that is disposed between the outside beams. It is advantageous to increase a correction force applied to the outside electron beams of the three in-line electron beams relative to the force applied to the central electron beam. Conventional electron guns adopt quadrupole lenses, the operation of which is described below, to adjust the length of focus and compensate for the distortion of an electron beam.
When a dynamic voltage applied to an electrode is increased as an electron beams emitted from a triode portion of the electron gun pass a pre-focus point and arrive at a quadrupole lens, the electron beams move in an electric field direction of the quadrupole lens. The electrons of the electron beams receive a divergent force in the vertical direction and a focusing force in the horizontal direction. Thus, when the beams are deflected toward the peripheral portion of a screen, the distortion of the electron beams due to the deflection electric field, the incident angle of the electron beams, and the curvature of a surface of the screen, is compensated. However, when the dynamic voltage applied to the electrode forming the quadrupole lens increases, the focal lengths and the distortions of each of the electron beams, among the three in-line electron beams, are different, because of corrections due to the diameter of a large diameter electron lens and a difference in the magnifying power of the outside electron beams as compared to the central electron beam.
Electron guns correcting astigmatism of an electron beam deflected toward the peripheral portion of a screen are disclosed in U.S. Pat. No. 4,701,677, U.S. Pat. No. 4,814,670, and U.S. Pat. No. 5,027,043.
Electron guns described in these publications include a means for converting an electron beam from a linear path, including a quadrupole lens, to correct astigmatism with a self convergence deflection yoke. The quadrupole lens has different voltages applied to electrodes where vertically elongated electron beam passing holes or horizontally elongated electron beam passing holes are located. This electron gun can converge the three in-line electron beams at one point and correct distortion of the beam due to vertical and horizontal deflection magnetic fields deflecting of the electron beam.
As the surface of a CRT becomes flatter and the deflection angle of electron beams increases, the difference in the focal lengths between the central and peripheral portions of a screen increases toward the periphery. Astigmatism of the electron beams at the periphery of the screen is thus produced by the deflection yoke. Therefore, an electron gun needs strong astigmatism and focal length correction forces to avoid loss of resolution at the periphery of the screen.
To obtain the strong astigmatism and focal length correction forces, a large difference in electric potential between electrodes forming the quadrupole lens and, accordingly, a high voltage, is needed. However, the high voltage may cause a problem in circuit reliability and withstand voltage between the electrodes of an electron gun. Also, when an electron beam is incident on the periphery of a screen, the incident angle of the beam decreases horizontally and increases vertically due to the function of the quadrupole lens adjacent to the main lens, that is, focusing the beam horizontally and diverging the beam vertically. Thus, the horizontal dimension of a spot at the periphery of a screen increases. The astigmatism varying with the deflection of an electron beam becomes serious as the deflection angle of the electron beam by the deflection yoke increases. Also, convergence is deteriorated.
A color CRT having a quadrupole lens compensating for these problems is disclosed in U.S. Pat. No. 6,051,919. In this CRT, the length of a plate of each plate electrode, at surfaces forming a quadrupole lens and facing each other, is different. The length at the central electron beam is longer than at the outside electron beams. Thus, the central electron beam has a stronger focus correction for correcting convergence and astigmatism at the peripheral of a screen than the outside electron beams. However, in the CRT having this quadrupole lens, as the deflection angle of an electron beam increases, the dynamic voltage applied to the electrode of the quadrupole lens is increased. As the dynamic voltage increases, the difference in the astigmatism correction of the central electron beam and of the side electron beams increases at the periphery of a screen, so that the focus property deteriorates.
In particular, this differential astigmatism correction phenomenon occurs severely in an electron gun with a large diameter electrode and a main lens. That is, when a dynamic voltage synchronized with a deflection signal is applied to the large diameter electrode, since the ratio of change in focusing of an electrostatic lens in vertical and horizontal directions for the central electron beam is greater than that for each of the side electron beams, the differential astigmatism correction is severe. This phenomenon occurs because equipotential lines 2 (see FIG. 2) are gradually distributed in the horizontal direction, compared to equipotential lines 1 (see FIG. 1) in the vertical direction, in an area through which the central electron beam passes, as shown in FIGS. 1 and 2. Thus, the effective diameter of the electrostatic lens in the horizontal direction is greater than in the vertical direction. When the strength of the main lens changes in response to a change in the dynamic voltage, the rate of change in the vertical direction is greater than in the vertical direction.
However, since the side electron beams are positioned at the side of the large diameter lens, when the dynamic voltage is changed, the effect of the change of the equipotential lines in the horizontal direction is greater at the central portion. Since the shape of the equipotential lines changes simultaneously for all electron beams in the horizontal direction, the rate of vertical elongation of the side electron beams is less than the rate of elongation of the central electron beam. Therefore, as shown in FIG. 3, the dynamic voltage to deflect the side electron beams toward the periphery of a screen needs to be higher voltage than the dynamic voltage applied to deflect the central electron beam.
Consequently, in an electron gun with a large diameter main lens, when the dynamic voltage is applied, to obtain high resolution at both the central portion and the periphery of a screen, a higher voltage needs to be applied to the side electron beams or a stronger quadrupole lens needs to be provided for the side electron beams.
To solve the above-described problems, it is an object of the present invention to provide an electron gun for a color CRT which provides a uniform electron beam spot throughout the entire fluorescent film by correcting astigmatism and improving the focusing property with a deflection yoke for the three electron beams landing on the periphery of a fluorescent film, as the deflection angle increases.
According to a first aspect of the invention, an electron gun for a color CRT includes a triode portion including cathodes for emitting electron beams, a control electrode, and a screen electrode; first and second focusing electrodes located on a common axis with the triode portion for forming a quadrupole lens; and a final focusing electrode forming a large diameter lens with the focusing electrodes and including an opening through which three electron beams commonly pass, the three electron beams lying in a horizontal plane and including a central electron beam and two side electron beams on opposite sides of the central electron beam, wherein the first and second focusing electrodes include a correction unit providing a correction force acting on the three electron beams and that is larger for the two side electron beams than for the central electron beam when a dynamic voltage, synchronized with a deflection signal, is applied to at least one of the first and second focusing electrodes for forming the quadrupole lens.
Further, in an electron gun according to the invention, each of the first and second focusing electrodes include plates facing each other, each plate having three circular electron beam passing holes, including a central electron beam passing hole and two side electron beam passing holes on opposite sides of the central beam passing hole, and the correction unit comprises on the first focusing electrode, respective vertical blades extending from the plate of the first focusing electrode toward the second focusing electrode and located adjacent each of the electron beam passing holes, wherein the vertical blades located adjacent the side electron beam passing holes, but not adjacent the central beam passing hole, extend closer to the second focusing electrode than other vertical blades; and on the second focusing electrode, respective horizontal blades extending from each of upper and lower horizontal sides of the plate of the second focusing electrode and being electrically longer at positions aligned with and corresponding to the two side electron beam passing holes than at a position aligned with and corresponding to the central electron beam passing hole.
According to another aspect of the invention, an electron gun for a color CRT includes, a triode portion including cathodes for emitting electron beams, a control electrode, and a screen electrode; first and second focusing electrodes located on a common axis with the triode portion; third and fourth focusing electrodes for forming a quadrupole lens; and a final focusing electrode adjacent the fourth focusing electrode, forming a large diameter lens, and including an opening through which three electron beams commonly pass, the three electrode beams lying in a horizontal plane and including a central electron beam and two side electron beams on opposite sides of the central electron beam wherein the first and second focusing electrodes include a correction unit providing a correction force acting on the three electron beams and that is larger for the two side electron beams than for the central electron beam when a dynamic voltage, synchronized with a deflection signal, is applied to the fourth electrode for forming the quadrupole lens.
According to a third aspect of the invention, an electron gun for a color CRT includes a triode portion including cathodes for emitting electron beams, a control electrode, and a screen electrode; first and second focusing electrodes located on a common axis with the triode portion; third, fourth, and fifth focusing electrodes for forming a quadrupole lens; and a final focusing electrode adjacent the fifth focusing electrode, forming a large diameter lens, and including an opening through which three electron beams commonly pass, the three electrode beams lying in a horizontal plane and including a central electron beam and two side electron beams on opposite sides of the central electron beam wherein the third, fourth, and fifth focusing electrodes include a correction unit providing a correction force acting on the two side electron beams and that is larger for the two side electron beams than for the central electron beam when a dynamic voltage, synchronized with a deflection signal, is applied to at least one of the third and fifth focusing electrodes for forming the quadrupole lens.